The present invention relates to a method of intervention at a number of distinct intervention points spread across the sea bed from the free end of one same riser originating from a semi-submersible platform designed for producing oil from a deposit.
Semi-submersible platforms are intended for oil production in very deep seas or oceans. They comprise a hull supported by legs, the bottoms of which are connected to a hollow base. The base and the buoyancy boxes in the legs provide the platform with buoyancy and stability. The hull, fixed on the legs, is kept above the surface of the sea while the installation is in production.
In order to collect extracted hydrocarbons, the platform is connected to the sea bed by several risers. These are connected to a collection of wells drilled vertically beneath the platform.
These wells are generally spread around the periphery of a central point, for example on a circle with a diameter of about 40 m.
Before the risers for producing oil from the deposit are installed, the various wells to which the production risers will be connected have to be drilled.
To carry out this drilling, the platform is equipped with a single drilling riser. This comprises an outer tube generally 20 inches (50.8 cm) in diameter through which the drilling members, such as a drill bit, are routed.
The wells are distributed around the periphery of a storage point provided on the sea bed at the center of the deposit. The storage point is generally defined by the term xe2x80x9cparking slotxe2x80x9d. It comprises a connector installed on the sea bed which allows the riser to be temporarily immobilized. The wells are bored one after another around the parking slot.
To do this, the platform is moved from one drill hole to the next, so that the top of the drilling riser is located vertically above the bore hole being produced.
Moving the platform in order to drill the various wells constitutes a lengthy and tricky operation because the platform has to be untethered, moved and then re-tethered over the chosen intervention point after each drilling operation.
The object of the invention is to propose a method of intervention on the sea bed using a riser which allows several wells to be bored or oil produced from them quickly and easily from one and the same platform.
To this end, the subject of the invention is a method of intervention at a number of distinct intervention points spread across the sea bed from the free end of a single riser originating from a semisubmersible platform designed for producing oil from a deposit, characterized in that it comprises the following successive steps:
a) tethering the platform to immobilize it in a position floating over the deposit;
b) installing the riser connecting the floating platform to the sea bed for intervention at a first intervention point;
c) carrying out an intervention at the first intervention point;
d) disengaging the free end of the riser from the first intervention point, while keeping this end submerged;
e) moving the free end of the riser with respect to the sea bed towards a second intervention point, while keeping the floating platform in place; and
f) carrying out an intervention at the second intervention point.
According to particular embodiments, the method comprises one or more of the following features:
step e) of moving the free end of the riser comprises the steps:
e1) installing at least one fixed point on the sea bed beforehand;
e2) installing hauling means between the free end of the riser and the or each fixed point; and
e3) hauling the free end of the riser using the hauling means connected to the or each associated fixed point, so as to cause the free end to move with respect to the sea bed towards the second intervention point;
there are three non-aligned distinct fixed points, to each of which are connected the hauling means allowing the free end of the riser to be moved under the action of three non-coaxial forces;
to implement step e), the method involves a step which consists in connecting the free end of the riser to at least one tether, the other end of which is fixed to the sea bed at points such that the first and second intervention points lie in the or each sector of a disc swept by the or each tether;
the free end of the riser is connected to two tethers which are fixed to the sea bed on each side of the segment delimited by the first and second intervention points, the length of the tethers being chosen such that the segment extends essentially along the axis of the biconvex crescent defined by the intersection of the two disc sectors swept by the tethers;
the hauling means are taken down to the bottom by a submersible, and once on the bottom, they are connected, on the one hand, to the or each associated fixed point and, on the other hand, to the free end of the riser;
the or each fixed point is carried by a vehicle capable of moving along the sea bed, which vehicle comprises means of tethering it to the sea bed, and, in order to install the or each fixed point:
a) the vehicle is taken along the bottom some distance away from the point adopted for installing the fixed point;
b) the vehicle is moved along the bottom as far as the installation point adopted; and
c) the vehicle is tethered to the installation point adopted.
xe2x80x83the or each fixed point is installed on the bottom before the platform is tethered over the deposit, the or each fixed point being installed underneath the area occupied by the platform;
xe2x80x83several fixed points connected to each other by ties extending along the sea bed between two anchors are installed, which ties are arranged around that region on the sea bed in which all the intervention points lie;
xe2x80x83means for returning the free end of the riser are installed between the riser and the sea bed;
xe2x80x83the return means are designed to return the free end of the riser towards the center of the number of intervention points; and
xe2x80x83the return means comprise a cable sliding in a sheath running along the riser, to which it is secured, which cable connects the sea bed to the platform, and the free end of the riser is returned towards the point at which the cable is connected to the sea bed by hauling the cable in the sheath from the platform.